translator_use_goto_tb() refuses to chain unless the destination is on the
same page as the start of the TB. For guests whose text is much larger than
a page this is expensive: an emulated alpha gcc compiling a 255k line
translation unit takes the indirect dispatch path for 8.4 billion of its
34.2 billion TB exits, and a large share of those are ordinary direct
branches that simply crossed an 8 KiB page boundary.
The restriction was made unconditional by d3a2a1d803 ("accel/tcg:
Introduce translator_use_goto_tb"), whose rationale was:
Various targets avoid the page crossing test for CONFIG_USER_ONLY,
but that is wrong: mmap and mprotect can change page permissions.
That is true, but in user-only builds the invalidation path already covers
it. There are no page tables: every mmap, mprotect and munmap reaches
page_set_flags(), which calls tb_invalidate_phys_range() whenever the flags
actually change, and tb_phys_invalidate() calls tb_jmp_unlink() to reset
incoming jumps. A chained cross-page jump is therefore broken whenever the
destination page's permissions change. This is not true in system mode,
where TBs are keyed by physical address and a page table change invalidates
nothing, so the restriction is kept there.
The rule protects one more thing, which the original rationale does not
mention: it guarantees that execution cannot enter a page without a TB
lookup, and so without check_for_breakpoints(). That is what makes a
breakpoint set after a block was translated take effect, since insertion
deliberately invalidates nothing. A link established before the breakpoint
was set would jump straight over it.
So the chaining is only enabled for a run that can never acquire a
breakpoint. In user-only mode every breakpoint comes from gdb -- BP_CPU is
g_assert_not_reached() there, and the guest cannot ask for one -- and gdb
has to be requested with -g before the first block is translated, even
though with suspend=n it may connect later. gdb_may_set_breakpoints()
reports whether it was, and is fixed for the lifetime of the process.
Add tests/tcg/alpha/test-xpage-chain.c to cover both hazards directly. It
places a direct branch near the end of one page targeting the next page,
runs it 200000 times so the chain is established, then checks that
mprotect(PROT_NONE) makes the next call fault, and that remapping the page
with different code runs the new code rather than a stale translation.
The test detects the hazard it is meant to detect: with the
tb_invalidate_phys_range() call in page_set_flags() commented out, it fails
both phases, executing page B after PROT_NONE and returning the stale
result.
Run with -b, the same binary stops once the chain is established and lets
tests/tcg/alpha/gdbstub/xpage-bp.py set a breakpoint on the far side of it,
which the next call has to stop on. With gdb_may_set_breakpoints() forced to
false so that the chaining stays on under gdb, that breakpoint is missed and
the test fails, which is what makes it a test of the gate rather than of
gdb.
Measured with qemu-alpha running an emulated alpha gcc 16.2.0 compiling the
SQLite 3.45.1 amalgamation on an x86-64 host, LTO build, on top of the
preceding patches:
before: 916,415,123,244 instructions
after: 891,254,240,071 instructions -2.75%
before: 85.59s wall clock
after: 81.45s wall clock -4.84%
Note that this is worth more in time than in instructions, the reverse of
the preceding patch: a chained jump replaces a cache probe whose loads can
miss, so the instructions it removes are more expensive than average.
Measured before the inline jump cache probe, when a missed chain cost a
helper call rather than an inline probe, the same change was worth -7.9%.
RFC because this reverses a deliberate decision and the reasoning above
wants review from someone who knows the invalidation paths better than I
do.
v3: Only take the shortcut when no gdbstub was requested. The same-page
rule also forces a lookup, and so a breakpoint check, on entry to every
page; without that, a chain established before a breakpoint was set runs
past it. Reported by Richard Henderson.
v3: Change translator_use_goto_tb() rather than translator_is_same_page().
i386, riscv and s390x call translator_is_same_page() for something else
-- enforcing that only a single-insn TB may cross a page -- and v2
changed their TB boundaries in user-only mode as a side effect. alpha
does not call it, so the numbers above are unaffected.
v3: Add the gdbstub half of the test.
Signed-off-by: Matt Turner <[email protected]>
---
accel/tcg/translator.c | 33 ++++++-
gdbstub/user.c | 14 +++
include/gdbstub/user.h | 11 +++
tests/tcg/alpha/Makefile.target | 17 +++-
tests/tcg/alpha/gdbstub/xpage-bp.py | 34 +++++++
tests/tcg/alpha/test-xpage-chain.c | 144 ++++++++++++++++++++++++++++
6 files changed, 251 insertions(+), 2 deletions(-)
create mode 100644 tests/tcg/alpha/gdbstub/xpage-bp.py
create mode 100644 tests/tcg/alpha/test-xpage-chain.c
diff --git ./accel/tcg/translator.c ./accel/tcg/translator.c
index 6c8fcd7a20..8879cd626f 100644
--- ./accel/tcg/translator.c
+++ ./accel/tcg/translator.c
@@ -15,6 +15,9 @@
#include "accel/tcg/cpu-mmu-index.h"
#include "exec/target_page.h"
#include "exec/translator.h"
+#ifdef CONFIG_USER_ONLY
+#include "gdbstub/user.h"
+#endif
#include "exec/plugin-gen.h"
#include "tcg/tcg-op-common.h"
#include "internal-common.h"
@@ -110,6 +113,34 @@ bool translator_is_same_page(const DisasContextBase *db,
vaddr addr)
return ((addr ^ db->pc_first) & TARGET_PAGE_MASK) == 0;
}
+/*
+ * Whether a direct jump may be chained to a destination outside the page
+ * the TB started in.
+ *
+ * In user-only mode there are no page tables. Every mmap, mprotect and
+ * munmap goes through page_set_flags(), which calls tb_invalidate_phys_range()
+ * whenever the flags actually change, and tb_phys_invalidate() unlinks
+ * incoming jumps. A cross-page link is therefore broken whenever the
+ * destination page's permissions change.
+ *
+ * What the same-page rule also provides is that execution cannot enter a page
+ * without a TB lookup, and so without check_for_breakpoints(), which is what
+ * makes a breakpoint set after a block was translated take effect. Nothing
+ * invalidates on breakpoint insertion, so a link established beforehand would
+ * jump straight over it. In user-only mode breakpoints only ever come from
+ * gdb -- BP_CPU is g_assert_not_reached() there and the guest has no way to
+ * ask for one -- and gdb has to be requested with -g before the first block
+ * is translated, so a run that has no gdbstub can never acquire a breakpoint.
+ */
+static bool use_cross_page_goto_tb(void)
+{
+#ifdef CONFIG_USER_ONLY
+ return !gdb_may_set_breakpoints();
+#else
+ return false;
+#endif
+}
+
bool translator_use_goto_tb(DisasContextBase *db, vaddr dest)
{
/* Suppress goto_tb if requested. */
@@ -118,7 +149,7 @@ bool translator_use_goto_tb(DisasContextBase *db, vaddr
dest)
}
/* Check for the dest on the same page as the start of the TB. */
- return translator_is_same_page(db, dest);
+ return use_cross_page_goto_tb() || translator_is_same_page(db, dest);
}
void translator_loop(CPUState *cpu, TranslationBlock *tb, int *max_insns,
diff --git ./gdbstub/user.c ./gdbstub/user.c
index 9e6f9a6f37..d810f0f38c 100644
--- ./gdbstub/user.c
+++ ./gdbstub/user.c
@@ -470,6 +470,18 @@ static void *gdbserver_accept_thread(void *arg)
#define USAGE "\nUsage: -g {port|path}[,suspend={y|n}]"
+/*
+ * Set before the guest runs and never cleared, so that code translated at
+ * any point can rely on it: with suspend=n gdb may connect long after
+ * startup, and once connected it can insert a breakpoint at any time.
+ */
+static bool gdbserver_requested;
+
+bool gdb_may_set_breakpoints(void)
+{
+ return gdbserver_requested;
+}
+
bool gdbserver_start(const char *args, Error **errp)
{
g_auto(GStrv) argv = g_strsplit(args, ",", 0);
@@ -513,6 +525,8 @@ bool gdbserver_start(const char *args, Error **errp)
return false;
}
+ gdbserver_requested = true;
+
if (suspend) {
if (gdbserver_accept(port, gdb_fd, port_or_path)) {
gdb_handlesig(first_cpu, 0, NULL, NULL, 0);
diff --git ./include/gdbstub/user.h ./include/gdbstub/user.h
index 654986d483..c091cd9758 100644
--- ./include/gdbstub/user.h
+++ ./include/gdbstub/user.h
@@ -11,6 +11,17 @@
#define MAX_SIGINFO_LENGTH 128
+/**
+ * gdb_may_set_breakpoints() - whether a breakpoint can ever be inserted
+ *
+ * In user-only mode every breakpoint comes from gdb, and gdb is only ever
+ * reachable if -g was given at startup, before the guest ran a single
+ * instruction. A run that has no gdbstub can therefore never acquire a
+ * breakpoint, which lets translation take shortcuts that a breakpoint
+ * would invalidate. Stays true once true, even if gdb detaches.
+ */
+bool gdb_may_set_breakpoints(void);
+
/**
* gdb_handlesig() - yield control to gdb
* @cpu: CPU
diff --git ./tests/tcg/alpha/Makefile.target ./tests/tcg/alpha/Makefile.target
index 36d8ed1eae..1a3f541bec 100644
--- ./tests/tcg/alpha/Makefile.target
+++ ./tests/tcg/alpha/Makefile.target
@@ -5,7 +5,7 @@
ALPHA_SRC=$(SRC_PATH)/tests/tcg/alpha
VPATH+=$(ALPHA_SRC)
-ALPHA_TESTS=hello-alpha test-cond test-cmov test-ovf test-cvttq
+ALPHA_TESTS=hello-alpha test-cond test-cmov test-ovf test-cvttq
test-xpage-chain
TESTS+=$(ALPHA_TESTS)
test-cmov: EXTRA_CFLAGS=-DTEST_CMOV
@@ -16,3 +16,18 @@ test-cmov: test-cond.c
test-plugin-mem-access: CFLAGS+=-mbwx
run-test-cmov: test-cmov
+
+ifneq ($(GDB),)
+GDB_SCRIPT=$(SRC_PATH)/tests/guest-debug/run-test.py
+
+# The chaining this exercises is only enabled when no gdbstub was requested,
+# so what is under test here is that requesting one turns it back off.
+run-gdbstub-xpage-bp: test-xpage-chain
+ $(call run-test, $@, $(GDB_SCRIPT) \
+ --gdb $(GDB) \
+ --qemu $(QEMU) --qargs "$(QEMU_OPTS)" \
+ --bin "$< -b" --test $(ALPHA_SRC)/gdbstub/xpage-bp.py, \
+ breakpoint behind an established cross-page chain)
+
+EXTRA_RUNS += run-gdbstub-xpage-bp
+endif
diff --git ./tests/tcg/alpha/gdbstub/xpage-bp.py
./tests/tcg/alpha/gdbstub/xpage-bp.py
new file mode 100644
index 0000000000..f0ec14cdec
--- /dev/null
+++ ./tests/tcg/alpha/gdbstub/xpage-bp.py
@@ -0,0 +1,34 @@
+"""Test that a breakpoint set after a cross-page chain is established is hit.
+
+translator_use_goto_tb() lets a direct branch chain to another page in
+user-only builds, which is only safe because a run with no gdbstub can never
+acquire a breakpoint. This runs with one, so the chaining must be off and
+the breakpoint must still be reached.
+
+This runs as a sourced script (via -x, via run-test.py).
+
+SPDX-License-Identifier: GPL-2.0-or-later
+"""
+from test_gdbstub import main, report
+
+
+def run_test():
+ """Run through the tests one by one"""
+ gdb.Breakpoint("break_here")
+ gdb.execute("continue")
+
+ # The chain exists by now; put a breakpoint on the far side of it.
+ target = int(gdb.parse_and_eval("(unsigned long)page_b_entry"))
+ gdb.execute("break *{}".format(target))
+ gdb.execute("continue")
+
+ pc = int(gdb.parse_and_eval("(unsigned long)$pc"))
+ report(pc == target, "stopped at {:#x}, expected {:#x}".format(pc, target))
+
+ gdb.execute("delete")
+ gdb.execute("continue")
+ exitcode = int(gdb.parse_and_eval("$_exitcode"))
+ report(exitcode == 0, "{} == 0".format(exitcode))
+
+
+main(run_test)
diff --git ./tests/tcg/alpha/test-xpage-chain.c
./tests/tcg/alpha/test-xpage-chain.c
new file mode 100644
index 0000000000..23b916ffe1
--- /dev/null
+++ ./tests/tcg/alpha/test-xpage-chain.c
@@ -0,0 +1,144 @@
+/*
+ * Cross-page TB chaining hazard test.
+ *
+ * Phase 1: a direct branch (br) near the end of page A targets page B.
+ * Run it enough times that QEMU chains TB_A -> TB_B.
+ * Phase 2: mprotect page B away. Re-running must fault.
+ * Phase 3: remap page B with different code. Re-running must execute the
+ * NEW code, not a stale chained translation of the old code.
+ *
+ * With -b, phases 2 and 3 are replaced by a stop at break_here(), where the
+ * gdbstub test sets a breakpoint on page B -- after the chain exists -- and
+ * checks that re-running the chain still stops on it. See
+ * tests/tcg/alpha/gdbstub/xpage-bp.py.
+ *
+ * SPDX-License-Identifier: GPL-2.0-or-later
+ */
+#include <stdbool.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <setjmp.h>
+#include <signal.h>
+#include <sys/mman.h>
+#include <unistd.h>
+
+#define PS 8192
+
+static sigjmp_buf jb;
+/*
+ * Written by the SIGSEGV handler and read by main(), so it must not be
+ * cached in a register across the faulting call.
+ */
+static volatile sig_atomic_t caught;
+
+/* Where the branch lands, for the gdbstub test to set a breakpoint on. */
+unsigned int *page_b_entry;
+
+/* Somewhere for the gdbstub test to stop once the chain is established. */
+void __attribute__((noinline)) break_here(void)
+{
+ asm volatile ("");
+}
+
+static void segv(int sig)
+{
+ caught = 1;
+ siglongjmp(jb, 1);
+}
+
+/* lda $0, imm($31) -> v0 = imm */
+static unsigned int lda_v0(int imm)
+{
+ return 0x201F0000u | (unsigned short)imm;
+}
+
+int main(int argc, char **argv)
+{
+ bool bp_mode = argc > 1 && strcmp(argv[1], "-b") == 0;
+ struct sigaction sa;
+ int rc = 0;
+ unsigned char *m = mmap(NULL, 2 * PS, PROT_READ | PROT_WRITE | PROT_EXEC,
+ MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
+ if (m == MAP_FAILED) {
+ perror("mmap");
+ return 2;
+ }
+
+ unsigned char *pa = m, *pb = m + PS;
+ unsigned int *entry = (unsigned int *)(pa + PS - 64);
+ unsigned int *tgt = (unsigned int *)(pb + 16);
+
+ page_b_entry = tgt;
+
+ entry[0] = lda_v0(1);
+ long disp = ((long)tgt - ((long)&entry[1] + 4)) / 4;
+ entry[1] = 0xC3E00000u | (unsigned int)(disp & 0x1FFFFF); /* br $31,tgt */
+ tgt[0] = 0x6BFA8001u; /* ret */
+ __builtin___clear_cache((char *)m, (char *)m + 2 * PS);
+
+ long (*fn)(void) = (long (*)(void))entry;
+
+ for (int i = 0; i < 200000; i++) {
+ if (fn() != 1) {
+ printf("FAIL: phase 1 wrong result\n");
+ return 1;
+ }
+ }
+ printf("phase 1 ok (chained)\n");
+
+ if (bp_mode) {
+ /*
+ * The chain from page A to page B now exists. gdb puts a breakpoint
+ * on page_b_entry here; the call below has to stop on it rather than
+ * jump over it.
+ */
+ break_here();
+ if (fn() != 1) {
+ printf("FAIL: bp phase wrong result\n");
+ return 1;
+ }
+ printf("bp phase ok\n");
+ return 0;
+ }
+
+ memset(&sa, 0, sizeof(sa));
+ sa.sa_handler = segv;
+ sigemptyset(&sa.sa_mask);
+ if (sigaction(SIGSEGV, &sa, NULL) != 0) {
+ perror("sigaction");
+ return 2;
+ }
+ if (mprotect(pb, PS, PROT_NONE) != 0) {
+ perror("mprotect");
+ return 2;
+ }
+ if (sigsetjmp(jb, 1) == 0) {
+ fn();
+ printf("FAIL: phase 2 executed page B after mprotect(PROT_NONE)\n");
+ rc = 1;
+ } else if (!caught) {
+ printf("FAIL: phase 2 longjmp without entering the handler\n");
+ rc = 1;
+ } else {
+ printf("phase 2 ok (faulted)\n");
+ }
+
+ /* Phase 3: remap with different code, expect the new code to run. */
+ if (mprotect(pb, PS, PROT_READ | PROT_WRITE | PROT_EXEC) != 0) {
+ perror("mprotect back");
+ return 2;
+ }
+ tgt[0] = lda_v0(2);
+ tgt[1] = 0x6BFA8001u;
+ __builtin___clear_cache((char *)pb, (char *)pb + PS);
+
+ long r = fn();
+ if (r != 2) {
+ printf("FAIL: phase 3 returned %ld, expected 2 (stale chain)\n", r);
+ rc = 1;
+ } else {
+ printf("phase 3 ok (new code ran)\n");
+ }
+ return rc;
+}
--
2.54.0
